Such a circuit arrangement is described in the older, not previously published German Patent Application No. 195 10 055.7. Specifically this discusses an active binary sensor as the source for this binary signal. This sensor generates a square-wave signal based on two different current values or current thresholds. The frequency of this square-wave signal contains the information to be measured.
From the older, not previously published DE 44 34 34 180 A1 a circuit arrangement is known for evaluating a binary signal, thus including the output signal of an active binary sensor, in which a signal current proportional to the sensor current is obtained with the aid of a current-mirror circuit. This signal current is tapped by way of an ohmic resistor from a constant-voltage source, whereby a binary voltage signal proportional to the sensor output voltage is generated. The current-mirror circuit thus serves for converting the current signal of the sensor into a binary voltage signal which can then be further processed with small effort and above all with a low power requirement.
Active sensors of the type under discussion here can be employed, for instance as wheel sensors for determining the rotational behavior of one of the individual wheels of a motor vehicle. The wheel rotational behavior is a particularly important input parameter for motor vehicle control systems which are utilized, for instance, for antilocking protection, for control of drive slippage, vehicle stability, and so on.
These control systems or some of their functions are considered safety-critical, because in case of a defect, the braking function or driving stability may be compromised. Numerous monitoring measures, error displays and so on are therefore prescribed.
Particular value is placed on designing the terminal wires or terminal pins with which the controller of ABS is equipped and which serves for connecting the wheel sensors and other components or the associated circuits to be short-circuit resistant. Since the loads in a motor vehicle are generally connected to one terminal of the vehicle battery or current generator by way of the vehicle chassis, this means that the second individual terminals or terminal pins lead to the second terminal of the battery. This applies also to the terminal pins of the sensors.
Therefore, when connecting the sensor terminal pins to the chassis, a short-circuit or overload protection must be in place. This has the consequence in practice that the terminals leading to the battery or an individual terminal of the battery (the positive pole, for instance) must be directed by way of a current-limiting circuit, which upon the occurrence of a short circuit restricts the current to a permissible value.
It is also prescribed that several mutually independent terminals of the sensors be present, so that, in case of a sensor defect or a short circuit, the effect of this short circuit is limited and the other sensors or at least some of the other sensors remain functional. Consequently, each of the terminals to be protected must be equipped with a separate current limiter.
The invention overcomes the problems associated with reducing the required effort for short-circuit-proof layout of the terminal pins of a circuit arrangement of the type mentioned initially, without having to accept disadvantages of other type.
This problem can be solved by the present invention. The particularity of the circuit arrangement according to the invention is thus that in a circuit system with a ground connection, as found in motor vehicles, the current source used for evaluating the sensor signal is inserted between the battery or individual terminal and the sensor terminal and serves as a current limiter and thus as overload protection in case of a short circuit or shunt between the sensor pin and ground.
Due to the mode of connecting the sensor or current source according to the invention, the current limiter circuit, which previously had to be inserted between the terminal pin and the battery terminal, becomes superfluous. Beyond that, an individual protection of each sensor terminal against short circuits is achieved, because each sensor or each sensor evaluation circuit is equipped in any case with a separate current source for evaluating the sensor signal.
Using the current limiter of the present invention results in a significant savings. In contrast to conventional current limiters, the circuit arrangement according to the invention can be implemented very easily by an integrated circuit. This integrated circuit can then be protected in a very simple manner by a thermal fuse, also constructed and integrated on a semiconductor basis, against overload by excessively high loss power caused, for instance, by a continuing short circuit.
Several particularly advantageous embodiments of the circuit arrangement according to the invention are described in the subordinate claims.
Additional characteristics, advantages and application possibilities of the invention can be obtained from the following description of embodiments on the basis of the attached figures.